Dr. Monzer Kahf, a prominent economist and counselor states: This act is a permissible one in Islam. The example of it is to put some Sadaqah money in a box written on it Zakah in a mosque. It may sometimes be wise because it amounts to giving a signal to the agent (the mosque management in the example) who administers and distributes the collection that this money should be distributed only to the same kind of Zakah distribution. You know that Zakah can only be spent on the categories that are mentioned in the Qur’an (9: 60) while Sadaqah, like any other donation, unless designated to a specific objective, may be spent at the discretion of the person who manages it.”